Fixes for git ioemu tree from Christoph Egger

Fixes for git ioemu tree from Christoph Egger

 Make ioemu build w/o CONFIG_PASSTHROUGH
- Make ioemu build w/ TAPGIFNAME defined
- Make pci_emulation_add non-static as it is used in different files
- remove redundant variable re-declarations/re-definitions
- Make xenstore functions static. They have no prototypes and aren't used
  elsewhere
- Initialize xenpv_machine correctly (xen_init_pv didn't match what
  QEMUmachine expected)
- Misc compiler warning fixes

Signed-off-by: Christoph Egger <Christoph.Egger@amd.com>